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Showing content from https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.inattribute below:

InAttribute Class (System.Runtime.InteropServices) | Microsoft Learn

Indicates that data should be marshaled from the caller to the callee, but not back to the caller.

public ref class InAttribute sealed : Attribute

[System.AttributeUsage(System.AttributeTargets.Parameter, Inherited=false)]
public sealed class InAttribute : Attribute

[System.AttributeUsage(System.AttributeTargets.Parameter, Inherited=false)]
[System.Runtime.InteropServices.ComVisible(true)]
public sealed class InAttribute : Attribute

[<System.AttributeUsage(System.AttributeTargets.Parameter, Inherited=false)>]
type InAttribute = class
    inherit Attribute

[<System.AttributeUsage(System.AttributeTargets.Parameter, Inherited=false)>]
[<System.Runtime.InteropServices.ComVisible(true)>]
type InAttribute = class
    inherit Attribute

Public NotInheritable Class InAttribute
Inherits Attribute

Inheritance

Attributes

The following example shows how to apply the InAttribute and OutAttribute to a platform invoke prototype that passes an array as a parameter. The combination of directional attributes allows the caller to see the changes made by the callee.

using namespace System;
using namespace System::Runtime::InteropServices;

// Declare a class member for each structure element.

[StructLayout(LayoutKind::Sequential, CharSet = CharSet::Unicode)]
public ref class OpenFileName
{
public:
    int structSize;
    String^ filter;
    String^ file;
    // ...
};

private ref class NativeMethods
{
public:

    // Declare a managed prototype for the unmanaged function.

    [DllImport("Comdlg32.dll", CharSet = CharSet::Unicode)]
    static bool GetOpenFileName([In, Out]OpenFileName^ ofn);
};

void main() {}

using System.Runtime.InteropServices;
using System;

// Declare a class member for each structure element.
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public class OpenFileName
{
    public int structSize = 0;
    public string filter = null;
    public string file = null;
    // ...
}

internal static class NativeMethods
{
    // Declare a managed prototype for the unmanaged function.
    [DllImport("Comdlg32.dll", CharSet = CharSet.Unicode)]
    internal static extern bool GetOpenFileName([In, Out] OpenFileName ofn);
}

public class MainMethod
{
    static void Main()
    { }
}

Imports System.Runtime.InteropServices

' Declare a class member for each structure element.
<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)>
Public Class OpenFileName

    Public structSize As Integer = 0
    Public filter As String = Nothing
    Public file As String = Nothing
    ' ...

End Class

Friend Class NativeMethods
    ' Declare managed prototype for the unmanaged function.
    Friend Declare Unicode Function GetOpenFileName Lib "Comdlg32.dll" (
       <[In](), Out()> ByVal ofn As OpenFileName) As Boolean
End Class

Public Class App
    Public Shared Sub Main()

    End Sub
End Class

You can apply this attribute to parameters.

The InAttribute is optional. The attribute is supported for COM interop and platform invoke only. In the absence of explicit settings, the interop marshaler assumes rules based on the parameter type, whether the parameter is passed by reference or by value, and whether the type is blittable or non-blittable. For example, the StringBuilder class is always assumed to be In/Out and an array of strings passed by value is assumed to be In.

You cannot apply the InAttribute to a parameter modified with the C#-styled out keyword. To avoid confusing the In keyword in Visual Basic with the InAttribute, minus Attribute, use the <[In]> form with brackets around the attribute.

Combining the InAttribute and OutAttribute is particularly useful when applied to arrays and formatted, non-blittable types. Callers see the changes a callee makes to these types only when you apply both attributes. Since these types require copying during marshaling, you can use InAttribute and OutAttribute to reduce unnecessary copies.

For more information on the effect of InAttribute on marshaling behavior, see Directional Attributes.

Maps a set of names to a corresponding set of dispatch identifiers.

Retrieves the type information for an object, which can be used to get the type information for an interface.

Retrieves the number of type information interfaces that an object provides (either 0 or 1).

Provides access to properties and methods exposed by an object.

• OutAttribute
• StringBuilder
• Blittable and Non-Blittable Types

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